The performance of slab-on-grade foundations can be adversely effected by several factors. The primary cause of foundation distress where a residential building or commercial structure has been constructed on an expansive clay soil is the transfer of moisture across the perimeter grade beam. Such conditions generally occur as the result of low rainfall over long periods of drought, as the result of evaporation through uncovered soil, or from extraction of moisture from the soil through tree or large bush roots.
The failure of a foundation to perform the function for which it is intended is manifested in several ways. Cracks on interior and exterior walls are perhaps the most obvious signs of foundation shifting but these can often be misleading. Other, and perhaps more reliable signs of foundation distress, include separations between brick veneer and window frames, separations at the corners of facia or trim boards, misaligned door frames (doors that will not close, that sometimes open by themselves, or which do not fit squarely in their frames), separations between rafters and ridge beams, and sloping floors.
Whatever the cause or manifestation of the foundation failure, the condition must be corrected or eventually the structure may collapse or have to be condemned. Some of the remedial measures available to compensate for excessive foundation distress include moisture stabilization, foundation underpinning, driven precast piles, moisture barriers and mud pumping.
The use of drilled piers to underpin a foundation is perhaps the most common remedial measure in use today. In this procedure holes approximately eight feet by eight feet by three feet are dug at selected intervals under the foundation grade beam. An eight-inch to thirty-inch shaft is then drilled to a depth of between eight and twenty feet below the surface of the soil to a bearing soil. The bottom of the hole is flared out to give a larger bearing surface. Reinforcing steel is then placed in the hole and the hole is filled with concrete up to within approximately one foot of the foundation grade beam. After the concrete has cured for at least three days, expensive structural jacks are placed beneath the grade beam and the foundation is raised upward. When the desired height of the foundation has been achieved, the jacks are replaced with concrete blocks and steel shims. The number and location of the piers is extremely important. Where only one part of the foundation has deflected, only that portion need be leveled and supported, but there is the risk that later work may be required to level other parts of the foundation.
The underpinning technique has disadvantages including the high cost of the piers, disastrous results if the piers should fail, and the disruption of normal activities during their construction. Additionally, if proper maintenance is not applied, such as moisture maintenance, the foundation may have to be releveled on the piers by rejacking and placement of additional steel shims.